1. Field of the Invention
The present invention relates to a method for suppressing an interference noise in an acoustic system. The acoustic system comprises at least one microphone and at least one loudspeaker. The at least one microphone generates an input signal and the at least one loudspeaker generates an acoustic signal which partially feeds back to the at least one microphone.
In an acoustic system of the type described above, as may be provided, for example, by a hearing device, interference noises caused by feedback may occur. An acoustic feedback may occur as a result of the acoustic signal generated by the loudspeaker being partially detected by the microphone, thereby being reintroduced into the acoustic system. The input signal generated by the microphone is amplified in the acoustic system, so that within the closed loop which is formed by the loudspeaker, the acoustic signal generated by the loudspeaker, the microphone, and the signal processing unit within the acoustic system, a signal component is constantly amplified into a whistling interference noise via the feedback, if the amplification during the signal processing exceeds a certain limit value within the acoustic system.
Such interference noises may be reduced or even eliminated via so-called feedback suppression methods (feedback cancelers). For this purpose, according to the related art, adaptive feedback cancelation methods are often used, in which an adaptive filter having filter coefficients h models the time-dependent impulse response of the acoustic feedback path. A frequently used example of a rule for adapting the filter coefficients h is provided by the normalized least mean square algorithm (NLMS):h(k+1)=h(k)+μe*(k)×(k)/|x(k)|2.
Here, k is the discrete time index, x is the input into the system for canceling the feedback, e=m−c is the error signal, which is defined as the difference between the input signal m generated by the microphone and the compensation signal c for compensating for the feedback. μ is the increment via which the speed of the adaptation or convergence is controlled, and * denotes the complex conjugation.
In a realistic acoustic system, the input signal m is often initially digitized at a comparatively high sampling rate and is thereby converted into discrete-time sample values. Subsequently, a plurality of successive sample values, for example, 128, is combined into a so-called frame in each case. Within a frame, a spectral analysis of the input signal may be carried out at this point by means of Fourier transformation, based on the sample values forming the frame. For the generation or analysis of a subsequent frame, the window to be examined is shifted by several sample values, for example, 32, in the direction of the time axis, so that the windows of the sample values for each frame to be considered partially, significantly overlap for adjacent frames. In this case, the time index may be regarded as a frame index, wherein the adaptive filter may also be used in the frequency domain. In this case, the filter coefficients h are vectors whose entries correspond to each spectral sub-band. However, the application is not limited to this case. Further details may be found, for example, in S. Haykin, “Adaptive Filter Theory” (Englewood Cliffs, N.J.: Prentice-Hall, 1996) or T. v. Waterschoot & M. Moonen, “Fifty years of acoustic feedback control: state of the art and future challenges” (Proc. IEEE, Vol. 99, No. 2, February 2011, pp. 288-327).
It is a known problem that correlated input signals, as, for example, may be generated by picking up music or spoken language, may result in a divergence in an adaptive filter, which may result in an at least partial cancellation of a target signal. This may produce significantly perceptible signal artifacts in the output signal, resulting in a considerable degradation of the sound quality. The whistling interference noises generated via an acoustic feedback also have a high correlation in the relevant signals, in particular if a correlated target signal is present which is picked up and fed back after being reproduced by a loudspeaker. If an adaptive filter is used at this point for suppressing the interference noises thereby generated, signal components of the target signal may thus also be at least partially canceled during the suppression of the interference signal of the feedback, which has a negative effect on the sound quality of the output signal.